This is a sealed canister of the US issue NBC gas mask filter C2A1 with a 40mm NATO thread for most gas masks, but were issued with the M40, M42, and M45.  Since sealed NATO cartridges do not “expire”, we will not be sorting or investigating dates for you. The ones in the steel outer container are the older ones, but still good.  The ones in this listing are the more modern plastic outer containers.  Images show various years of US filters, yours would look like one of these.